The present invention relates to a distance measuring device for use in a camera employing an automatic focusing system and, more particularly, to an improvement in the distance measuring device employing the principle of triangulation using a light beam.
It is known in the art to use a light beam for measuring distance by triangulation. One example is schematically shown in FIG. 1 of the attached drawing and includes first and second stationary lens L1 and L2 positioned side-by-side and spaced a predetermined distance D from each other. Positioned behind the first lens L1 is a light source 1 for emitting a beam of light through the first lens L1 towards an object. A plurality, of, for example, four as shown, photoresponsive sensing elements 9, 10, 11 and 12 are arranged in a row behind the second stationary lens L2. A pulsed light beam 3 emitted from the light source 1 and passing through the lens L1 is directed to an object relative to which the distance from the device is to be measured. The light beam reflected from the object which may be located at any point along the path of travel of light beam 3 will, after having passed through the second stationary lens L2, impinge on and be properly focused on one of the photoresponsive elements 9, 10, 11 and 12. If the object is located at the position 4, the light beam reflected from the object will fall and be properly focused on the photoresponsive element 9. Accordingly, a pulse signal is produced from the photoresponsive element 9 indicating that the object is located at the position 4. This pulse signal is applied to a control circuit 13 which may be so operated upon receipt of the pulse signal from the element 9 as to control an optical arrangement, coupled thereto, to be brought in a focused condition with the object located at the position 4.
Similarly, the photoresponsive elements 10, 11 and 12 produce a pulse signal when the object is located at positions 5, 6 and 7, respectively.
In addition to the reflected light beam, each of the photoresponsive elements 9 to 12 tends to receive ambient light such as that emitted by incandescent and/or fluorescent lamps operating on alternating current, which often adversely affect the performance of each photoresponsive sensing element and, hence, the characteristics of a pulse signal. Generally, the lamp, for example, a fluorescent lamp or an incandescent lamp is lit by the supply of a commercial AC power source having a frequency of 60 Hz. Thus, although the human eyes are not able to perceive it, the light intensity from the lamp fluctuates at the frequency of a 120 Hz in a form of sinusoidal wave. FIG. 2 of the attached drawing shows a graph of light intensity in which the curve M represents the light intensity of ambient light, while the curve N represents the light intensity of received light beam. The total light intensity received by the photoresponsive element can be considered as a sum of the intensities represented by the curves M and N.
The intensity t' of the received light beam N varies with the distance between the light source 1 and the object and/or the reflection factor of the object, and the degree of fluctuation t varies with the brightness of the lamp. Therefore, there may be a case where the intensity t' of the received light beam N becomes smaller than the fluctuation t of the light from the lamp. Furthermore, the region or location on which the light beam N is added over the ambient light M depends on the moment when the light beam is radiated. For example, the light beam N can be radiated in a region K1, K2, K3 or K4 shown in FIG. 2. Since the signal obtained from each of the sensing elements is in relation to the light intensity, each of the photoresponsive elements produces a similar sinusoidal wave with a pulse signal corresponding to the curve N being impressed over the fluctuating signal. The detection of such pulse signal indicative of received light beam is normally carried out by the use of a comparator which compares the received signal from the photoresponsive element with a predetermined threshold level and produces a signal when the received signal exceeds the threshold level. Since the signal corresponding to the ambient light may fluctuate, it is quite difficult to distinguish the pulse signal particularly when the amplitude of the pulse signal is very small.
Although this difficulty can be removed by modulating the light beam into a particular frequency distinguishable from the frequency of the commercial AC power source, such modulation requires a considerably increased manufacturing cost because of the employment of the frequency modulator and also requires higher electric power for the operation.
Another method for removing the difficulty is to use light beam having a particular color and optical filter for filtering only the light beam. This method, however, has a disadvantage in that the selected particular color for the beam light may be interfered with by the same color light contained in the ambient light.